1. Field of the Invention
The present invention relates to a third-order nonlinear optical main chain polymer material and to a method for preparing the same.
2. Description of the Prior Art
When a material is irradiated with a light, the electric polarization, P, of the material induced by an optoelectric field, E, may generally be expressed by the general formula (1) below: EQU P=.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+ . . . (1)
where .chi..sup.(i) (i.gtoreq.2) is called an i-th nonlinear sensitivity. Second harmonic generation (SHG) and third harmonic generation (THG) due to second and third terms, respectively, are well known as wavelength conversion effects. The third term is also important as a factor giving rise to changes in optical constants, for example, nonlinear refractive index effects and nonlinear absorption coefficient effects. In particular, nonlinear refractive index effects relate to change in refractive index of a material which is in proportion to the intensity of an incident light, as represented by the equation (2) below: EQU n=n.sub.0 +n.sub.2 I (2)
wherein n.sub.0 is a refractive index at a weak light intensity, I is an intensity of incident light, and n.sub.2 is a nonlinear refractive index. Nonlinear refractive index, n.sub.2, and .chi..sup.(3) may be correlated by the equation (3) below: EQU n.sub.2 =(16 .pi..sup.2 /Cn.sub.0.sup.2).chi..sup.(3) ( 3)
wherein n.sub.0 and n.sub.2 are as defined above, and C is the velocity of light (electromagnetic constant).
Both n.sub.2 and .chi..sup.(3) may be used as indices each indicating a degree of nonlinear optical effect.
By combining a material having this effect with one or more other optical devices such as a Fabry-Perot etalon, a polarizer, and a reflector, it will be possible to realize nonlinear optical devices such as an optical bistable device, an optically gated device, and a phase resonance wave generator. These nonlinear optical devices are hopeful as key devices for future optical computing and optical conversion technology (on nonlinear optical device in general, reference is made to Conf. Lec. of IEEE Int. Conf. Commun, p. 1152 (1990)). Almost all the performances of optical nonlinear devices, i.e., range of wavelength used, strength of input light for operation, response speed, and the like depend on the characteristics of the materials constituting the devices as will be explained by examples below. With respect to details of comparison of materials for nonlinear optical devices, reference is made to Oyo Butsuri (Applied Physics) Vol. 59, p.155, (1990).
GaAs/GaAlAs semiconductor superlattice crystals are based on the mechanism that the refractive index changes depending on the intensity of light due to excitation of exciton as a result of absorption of light in the crystal (absorptional nonlinear effect). Therefore, they are highly effective and require only low incident light intensities for their operation. However, they have disadvantages that the range of wavelength which can be used is limited to a very narrow range near the absorption spectrum of the exciton and that the response time depends on the lifetime of exciton, thus failing to be used in optical signal processing higher than 3.times.10.sup.-8 sec.
Carbon disulfide (CS.sub.2) known as a nonlinear optical liquid is based on the mechanism that the refractive index shows a dependency on the intensity of light applied due to the arrangement of molecules permitting rotation dependent on the optoelectric field applied (molecular rotation nonlinear effect). This is advantageous in that the wavelength range of incident light which can be used is broad enough to cover from visible to near-infrared. However, not only their third nonlinear coefficient is low but also their time response depends on molecular relaxation times, and hence it is impossible to use them in optical signal processing faster than 10.sup.-11 to 10.sup.-12 sec. On the performances of CS.sub.2, reference is made to Applied Physics Letters, Vol. 15, p. 192 (1969).
While it has an efficiency by 10 digits lower than those of semiconductor superlattice crystals quartz can be made in the form of fiber to make an optical multiplexer/demultiplexer switch which can be operated with an incident light of several watts (W). However, the length of such quartz fiber must be from 100 to 1,000 m in order for it to be useful, and its response speed has remained at a level at most on the order of 10.sup.-10 sec. because of its increased length. On the details of optical switches using quartz fibers, reference is made to Denki Gakkai Tsushinshi OCS88-46, 37 (1988).
Therefore, there has been a keen demand for developing a material which has a wide wavelength range in which it can be used, has a high third order optical nonlinear efficiency, and enables high speed response on the order of pico second or less.
Among the materials exhibiting nonlinear optical effects, organic materials having .pi. electron conjugates which enable high speed response have been given attention. Specific examples of such organic materials include .pi. conjugated polymers such as polydiacetylene, polyacetylene, and polyarylenevinylene. The nonlinear optical effects of the organic materials having .pi. electron conjugates are purely due to electron polarization unlike semiconductors and dielectrics whose nonlinear optical effects are based on lattice-to-lattice interactions, resulting in a high response speed as high as 10.sup.-14 sec. which makes it possible to follow changes in the intensity of optical signal. For example, when poly(2,4-hexadiyne-1,6-(p-toluenesulfonate)) (abbreviated as "PTS"), one of polydiacetylenes, is used, the input wavelength range which can be used is from about 0.65 .mu.m to 2.0 .mu.m or more, the nonlinear refractive index (n.sub.2) is 2.times.10.sup.-12 (W/cm.sup.2).sup.-1 which is by 2 digits larger than that of the aforementioned CS.sub.2 liquid. Therefore, the organic materials having .pi. electron conjugates are most expected among various materials for realizing nonlinear optical devices. On nonlinear optical properties of PTS, reference is made to Physical Review Letters, Vol. 36, p. 956 (1976).
However, many of the .pi. conjugated polymers having large .chi..sup.(3) values are insoluble and unmeltable, and hence are poorly processable. Even when they can be formed into films, the resulting films have low optical transmittivities because of their rigidity and crystallinity and are poor in their processability into desired optical waveguides and it has been difficult to use them as they are for fabricating various devices. In fact, no nonlinear optical device composed of PTS referred to above having the largest .chi..sup.(3) has been realized yet. For the same reason, no device using polyacetylene or polyallylenevinylene has been developed yet.
On the other hand, organic materials having large nonlinear optical effects other than .pi. conjugated polymers include donor-acceptor type .pi. conjugated molecules. This type of molecule has a relatively short .pi.-conjugated system with one end thereof substituted with a donor and the other with an acceptor, and the molecule is intended to amplify nonlinear optical effects utilizing intramolecular charge transfer effects generated between the donor and the acceptor. More specifically, diethylaminonitrostilbene (hereafter, abbreviated as DEANS; cf. Chemical Physics Letters, Vol. 165, p. 171 (1990)) and diethylaminonitrostyrene (hereafter, abbreviated as DEANST; cf. U.S. Pat. No. 4,997,595 (1991)) have been known. The donor and acceptor in the aforementioned compounds are a diethylamino group and a nitro group, respectively, while the .pi.-conjugated system is stilbene in DEANS and styrene in DEANST. The both compounds have .chi..sup.(3) values on the order of at most 10.sup.-12 to 10.sup.-11 esu. Particularly, it has been tried to make an optically gated device using a nitrobenzene solution of DEANST which is a nonlinear optical medium superior over CS.sub.2, and test its performances. Further, a side chain type polymer is known which consists of a donor-acceptor type .pi.-conjugated molecule containing azobenzene as a .pi.-conjugated system and polymethyl methacrylate (hereafter, abbreviated as PMMA) to which the donor-acceptor type .pi.-conjugated molecule is bonded through a covalent bond as a side chain to form a side chain type polymer and endow it with a desired optical transmittivity. For the first example of nonlinear optical side chain type polymer, reference is made to Applied Physics Letters, Vol. 51, p. 1 (1987). However, the nonlinear optical side chain type polymer has a disadvantage that it has a .chi..sup.(3) value by at least one digit smaller than that of the .pi.-conjugated polymer.
Therefore, in order to realize high speed nonlinear optical devices using organic materials, it is essential to develop a new organic material having a .chi..sup.(3) value as large as .pi.-conjugated polymers and having an acceptable processability and optical transmittivity. To achieve semiconductor laser behavior, it has been desired that .chi..sup.(3) should be at least 10.sup.-10 esu, and 10.sup.-12 (W/cm.sup.2).sup.-1 as expressed in terms of n.sub.2 represented by formula (3) above.
As described above, most of the organic materials having .chi..sup.(3) no smaller than 10.sup.-10 esu are .pi.-conjugated polymers, which are rigid and of high crystallinities, resulting in poor processabilities and low optical transmittivities, thus failing to give sufficient processabilities to desired waveguide structures. On the other hand, nonlinear optical side chain type polymer materials have similar disadvantages that the donor-acceptor type .pi.-conjugated molecule giving rise to optical nonlinearity has a .chi..sup.(3) value smaller than 10.sup.-10 esu, and it has been difficult to introduce the molecule in the polymer material in high concentrations. Nonlinear optical side chain type polymer materials can be produced by a radical copolymerization or a macromolecule reaction of a vinyl monomer having a .chi..sup.(3) component. In the radical copolymerization, nitro groups and azo bonds, which are indispensable for increasing the .chi..sup.(3) value of the resulting polymer, act as a radical inhibitor, and radical inhibition tends to occur with an increased content of .chi..sup.(3) component. As a result, the degree of polymerization remains at a low level, and the polymer obtained has a low film-formability. Hence, it has been difficult to obtain a material having a high concentration of .chi..sup.(3) component and an acceptably high processability. For example, the ratio of an introduced .chi..sup.(3) component which is disazo or more and has nitro groups and azo bonds in the molecule is at most 10 to 30 mol %. On the other hand, utilization of the macromolecule reaction results in a ratio of introduction of the .chi..sup.(3) component of at most about 20 mol %. In addition, the .chi..sup.(3) value of the nonlinear optical side chain type polymer material prepared by this macromolecule reaction is found to be at most on the order of about 10.sup.-11 esu.
As described above, conventional nonlinear optical side chain type polymer materials use polymers having high processabilities and high optical transmittivities such as PMMA as a base polymer and hence they have high potentialities of being applied to practical devices but they have a critical disadvantage that their .chi..sup.(3) values are by about one digit smaller than .pi.-conjugated polymers.